Objectives: The aim of the present study was to evaluate the effect of irradiation on de- and remineralisation of human dentin in vitro. Methods: The root surfaces of 15 freshly extracted caries-free human molars were cleaned, thereby removing the cementum. The teeth were then coated with acid-resistant nail varnish, exposing four rectangular windows. Two windows served as a non-irradiated control, while the other two windows were irradiated. The irradiation dose of 60 Gy was fractionally applied over six weeks (2 Gy/day). All specimens were distributed among the following experimental groups: A: non-irradiated, only demineralisation; AA: non-irradiated, de- and remineralisation; B: irradiated, only demineralisation; BB: irradiated, de- and remineralisation. All specimens were demineralized for 21 days with acidified gel. The remineralisation was performed using a calciumphosphate model for 12 days. From each tooth, two dentinal slabs were cut. The depth of the demineralized areas was determined using a polarized light microscope. Results: In the case of the non-irradiated specimens a mean lesion depth of 205 µm (± 51 µm, group A) after demineralisation and 178 µm (± 32 µm, group AA) after remineralisation was observed. The irradiated specimens showed a mean lesion depth of 195 µm (± 51 µm, group B) after demineralisation and 177 µm (± 42 µm, group BB) after remineralisation. Statistical analysis showed in both cases a significant decrease of the lesion depth after remineralisation (p Conclusion: Within the limitations of an in vitro investigation it can be concluded that irradiation has no effect on de- and remineralisation of human dentin.